1. Field of the Invention
This invention relates generally to methods for cleaning and etching silicon surfaces, and more particularly to using NF.sub.3 to pre-clean wafers at low temperatures.
2. Description of Related Art
During fabrication of semiconductor devices using silicon wafers, contaminants and impurities may form on the silicon surface of the wafer prior to various processing steps such as epitaxial silicon deposition or oxide layer growth. After removing the contaminants, if the wafer is exposed to oxygen, silicon atoms on the wafer surface immediately bond with the oxygen to form a thin SiO.sub.2 film (up to approximately 20 .ANG.) on the wafer surface. This native oxide, SiO.sub.2, may interfere with the deposition process, e.g., a polycrystalline film may form when epitaxial film deposition is desired, resulting, for example, in device noise or degraded performance. In addition, native oxide decreases the yield of production of semiconductor devices. The formation of native oxide and the presence of various contaminants on the silicon surface become increasingly serious problems as device geometries become smaller. Such native oxides degrade control and reproducability during the semiconductor device fabrication processes.
Therefore, the native oxide film and contaminants must be removed before deposition and growth of certain films during fabrication of the semiconductor device so that the silicon surface of the wafer is free of oxides and contaminants prior to epitaxial deposition. Conventional cleaning methods use the process chamber of a chemical vapor deposition (CVD) reactor for both cleaning the native oxide off the wafer and for wafer processing, i.e., the deposition of films onto the wafer.
One method, called a hydrogen bake, uses hydrogen to reduce the native oxide from the silicon layer. For a hydrogen bake, the wafer is brought to a high temperature, such as, for instance 1150.degree. C. Hydrogen gas is pumped into the chamber, which reacts with the wafer surface to reduce the native oxide from the silicon, thereby removing the native oxide. The chamber and/or wafer are then cooled to a temperature for silicon deposition.
Another cleaning method uses an HCl etch, oftentimes in conjunction with a hydrogen bake. In this method, the wafer is brought to temperature (i.e., 1150.degree. C. or more) in the reaction chamber. A conventional HCl etch (e.g., 1-5% HCl in H.sub.2) is then performed to remove damaged silicon and metal contamination from the wafer surface caused by prior processes, such as wafer polishing. As a result, both native oxide and damage from the silicon surface are removed. The temperature of the wafer is then lowered for epitaxial deposition or oxide growth.
These cleaning techniques require the wafer to be brought to a high temperature in the epitaxial deposition process chamber. This temperature is higher than what is usually required for epitaxial deposition. At high temperatures, the strength of silicon wafers decreases, causing defects such as slip, which can lead to increased yield loss.
Bringing the wafer to a high temperature within the process chamber may also result in undesirable autodoping. The wafer generally includes a semiconductor substrate, typically silicon, in which are formed n-type and p-type regions and on which are formed insulating and conductive layers. In the semiconductor fabrication process, heavily-doped portions in the substrate are often covered with a lightly-doped epitaxial layer in order to achieve a sharp junction between the heavily-doped substrate and the lightly-doped epitaxial layer. However, as the wafer temperature rises to the cleaning temperature, dopants from heavily doped regions evaporate and deposit on the chamber walls and on other parts of the wafer. After the wafer is cleaned, a lightly-doped epitaxial layer is deposited on the wafer. During this deposition, dopants on the chamber wall may dislodge and contaminate the epitaxial layer, resulting in unwanted and unpredictable changes in the dopant concentration in the epitaxial layer.
A further disadvantage with conventional cleaning methods using the process chamber is that throughput of the system is reduced due to the time taken to clean the wafer. Because cleaning and epitaxial deposition are both done in the process chamber, these processes must be performed sequentially. Throughput can be increased by adding more process chambers to the system. However, because process chambers are expensive, increasing throughput using more process chambers increases the cost of the system.
Accordingly, a method of cleaning a silicon surface is desired which overcomes the above-described deficiencies.